1. Field of the Invention
The present invention relates in general to a method and a system for controlling a mold clamping apparatus including a mold clamping device having a main cylinder chamber and an auxiliary cylinder chamber for rapidly opening and closing a mold and clamping the mold in the closed position with a high pressure. More particularly, the present invention is concerned with technologies for effectively shortening a cycle time of a forming operation using the mold, by shortening the time required to close and clamp the mold, and for improving the accuracy of control of the mold clamping force.
2. Discussion of the Related Art
As a device for clamping a mold used for an injection molding machine, for example, there is known a mold clamping device having a main cylinder chamber and an auxiliary cylinder chamber for moving a movable plate with a movable mold half fixed thereto toward a stationary plate having a stationary mold half fixed thereto, and a mold opening cylinder chamber for moving the movable plate away from the stationary plate. Those cylinder chambers are provided independently of each other. For instance, there are known a booster ram type mold clamping device and a side cylinder type mold clamping device. The mold clamping device of the booster ram type includes a main ram which defines the main cylinder chamber as a mold clamping chamber and a mold closing chamber on its opposite sides, and which has the auxiliary cylinder chamber as a mold closing chamber. The mold clamping device of the side cylinder type includes a main cylinder having the main cylinder chamber as a mold clamping chamber, and auxiliary cylinders which are disposed around the main cylinder and each of which includes a piston defining the auxiliary cylinder chamber as a mold closing chamber and a mold opening chamber on its opposite sides. The main cylinder chamber functioning as the mold clamping chamber has a larger effective pressure-receiving area than the auxiliary cylinder chamber functioning as the mold closing chamber. A mold clamping apparatus including such a mold clamping device includes a hydraulic device having pumping means for delivering a pressurized working fluid into the mold closing chamber to close the mold at a relatively high velocity, and a switch valve which is opened to apply the pressure of the working fluid to the mold clamping chamber as well as the mold closing chamber, to clamp the mold with a sufficiently large clamping force. A suitable material is injected into the closed and clamped mold, and the pressurized fluid is delivered into the mold opening chamber to open the mold, to remove an article formed by injection molding.
The hydraulic device of such a mold clamping apparatus uses a servo valve for controlling the mold closing and opening velocities and the mold clamping force. That is, the servo valve is capable of controlling the pressure and the rate of flow of the working fluid supplied to the mold closing, clamping and opening chambers. The hydraulic device preferably includes a surge valve connected to the mold clamping chamber (main cylinder chamber) which prevents the generation of a negative or reduced pressure in the mold clamping chamber when the mold is closed with the pressurized fluid being applied to the mold closing chamber, and prevents the pressure in the mold clamping chamber from being raised when the mold is opened with the pressurized fluid being applied to the mold opening chamber.
In the conventional mold clamping system, the switch valve is provided in a fluid passage connecting the servo valve and the mold closing chamber. Upon closing of the mold, the switch valve is opened so that the pressure of the pressurized fluid delivered from the servo valve to the mold closing chamber is also applied to the mold clamping chamber. Described more specifically, the switch valve is initially placed in the closed position, so that the pressurized fluid is supplied from the servo valve to only the mold closing chamber. When the movable plate with the movable mold half fixed thereto has been moved to a predetermined mold closing position, this position is detected by a position sensor, and the switch valve is opened to supply the pressurized fluid to the mold clamping chamber as well as the mold closing chamber. In the mold clamping operation, the hydraulic pressure to be applied to the mold clamping chamber is controlled in a feedback or closed-loop control fashion, so that the hydraulic pressure coincides with a predetermined value corresponding to a desired mold clamping force by which the mold is held in the closed position. For the closed-loop control of the mold clamping force, the pressure in the mold closing chamber detected by a pressure sensor is generally used as the pressure in the mold clamping chamber, since the pressurized fluid is supplied to the mold clamping chamber through the fluid passage connecting the servo valve and the mold closing chamber. The pressure to be applied to the mold clamping chamber as well as the mold closing chamber is controlled on the basis of the output signal of the pressure sensor.
Since the pressurized fluid is supplied to the mold clamping chamber through the switch valve connected to the fluid passage indicated above, the pressure in the mold clamping chamber is not raised to the level of the pressure in the mold closing chamber until the opening action of the switch valve is completed. Therefore, the initiation of the feedback control of the fluid pressure to control the mold clamping force should be delayed from the point of time at which the mold closing action is completed (at which the switch valve is commanded to be opened). An appropriate delay time after the completion of the mold closing action and before the initiation of the mold clamping force is measured by a timer or time counter, so that the control of the mold clamping force is initiated when the delay time has passed. Accordingly, the rise of the mold clamping force is delayed, and the required cycle time of a forming operation with the mold is undesirably increased.
Another problem exists in the presence of the switch valve between the mold clamping chamber and the fluid passage connected to the mold closing chamber. Namely, the pressure in the mold clamping chamber does not accurately coincide with the pressure in the mold closing chamber, due to pressure losses in the switch valve and fluid passage connecting the switch valve and the mold clamping chamber. Accordingly, the feedback control of the mold clamping force cannot be achieved with a sufficiently high degree of accuracy.